Preserving orthogonality between uplink, UL, transmissions from multiple user equipments, UEs, in a Long Term Evolution, LTE, network requires time alignment at the receiving radio base stations, which are denoted as “eNodeBs” or “eNBs” according to the Third Generation Partnership Project, 3GPP, lexicon. Correspondingly, a given eNB controls the UL transmit timing of the UEs operating under its control, to ensure that the signals transmitted by the UEs arrive at the eNB aligned in time, i.e., well within the cyclic prefix, CP. The so-called “normal” CP length in LTE is about 4.7 μs.
UL timing alignment ensures that the eNB can reliably use the same resources, such as Discrete Fourier Transform, DFT, or Fast Fourier Transform, FFT, resources, to receive and process UL signals incoming from multiple UEs. Timing Advance, TA, commands provide the mechanism by which the eNB controls the UL transmission timing of individual UEs, to ensure that the eNB receives the UL signals from the different UEs in time-aligned fashion. According to the TA mechanism, the eNB sends TA commands to a UE based on measurements on UL transmissions from that UE. For example the eNodeB measures two way propagation delay or round trip time for each UE, to determine the value of the TA required for that UE. For a TA command received by a UE on subframe n, the corresponding adjustment of the uplink transmission timing shall be applied by the UE from the beginning of subframe n+6. The TA command indicates the change of the uplink timing relative to the current uplink timing of the UE transmission as multiples of 16 Ts, where Ts=32.5 ns and is called basic time unit in LTE.
In case of random access response, an 11-bit timing advance command, TA, for a Timing Advance Group, TAG, indicates NTA values by index values of TA=0, 1, 2, . . . , 1282. Thus, the amount of the timing alignment for the TAG is given by NTA=TA×16. In other cases, a 6-bit timing advance command, TA, for a TAG indicates adjustment of the current NTA value, NTA,old, to the new NTA value, NTA,new, by index values of TA=0, 1, 2, . . . , 63, where NTA,new=NTA,old+(TA−31)×16. Here, adjustment of the NTA value by a positive or a negative amount indicates advancing or delaying the uplink transmission timing for the TAG by a given amount respectively. Timing advance updates are signaled by the eNB to the UE in Medium Access Control, MAC, Protocol Data Units, PDUs.
As a general rule, a UE performs inter-frequency and inter-RAT measurements in measurement gaps, unless it is capable of performing them without gaps. Here, “RAT” denotes “Radio Access Technology” and “inter-RAT” denotes measurements by a UE on a different radio technology, such as where the UE is operating in a E-UTRAN—E-UTRAN denotes Evolved Universal Terrestrial Radio Access Network—and measures one or more cells of a UTRAN. To enable inter-frequency and inter-RAT measurements for a UE that requires gaps, the network has to configure the measurement gaps.
Two periodic measurement gap patterns both with a measurement gap length of 6 ms are defined for LTE. The first pattern, denoted as measurement gap pattern #0, has a repetition period of 40 ms. The second pattern, denoted as measurement gap pattern #1, has a repetition period of 80 ms. Measurements performed by the UE during configured measurement gaps are reported to the network, which uses them for various tasks. The following measurements are specified or can be performed by a LTE-based UE during configured measurement gaps: inter-frequency cell detection or cell identification; inter-frequency Reference Signal Received Power, RSRP, measurements; inter-frequency Reference Signal Received Quality, RSRQ, measurements; inter-frequency Reference Signal Time Difference, RSTD, measurements; inter-RAT cell identification; and inter-RAT measurements, such as Common Pilot Channel or CPICH measurements, Reference Signal Code Power or RSCP measurements, Carrier to Interference measurements, received signal strength measurements, etc.
The measurement gaps are used in all duplex modes of operation, including Frequency Division Duplex or FDD, Time Division Duplex or TDD, and Half Duplex FDD, denoted as HD-FDD or simply HD. In HD operation, the UL and Downlink, DL, transmissions take place on different paired carrier frequencies but are not simultaneous in time in the same cell. This fact means that the UL and DL transmissions take place in different time resources, e.g. in different symbols, time slots, subframes or frames. In other words, the UL and DL subframes do not overlap in time. The number and location of subframes used for DL, UL or unused subframes can vary from frame to frame, or can vary across multiples of frames.
FIG. 1 illustrates a known frame structure as defined for E-UTRA TDD. In particular, the illustrated frame structure corresponds to the different UL/DL TDD configurations depicted in the table shown in FIG. 2. In the table, “U” denotes an UL subframe, “D” denotes a DL subframe, and “S” denotes a special subframe that is divided into a DL portion, DwPTS, and an UL portion, UpPTS, separated by a guard period, GP. Note that the measurements gaps having subframe offsets of 3 and 8 are squeezed in between two uplink subframes, for UL/DL configuration #0. FIG. 3 illustrates this circumstance. Measurement gaps with offsets of 2 and 7 subframes are squeezed in between a special subframe and an uplink subframe, for UL/DL configurations #0, #1 and #6. FIG. 4 illustrates these circumstances.
Regarding measurements performed by a UE in a LTE network in autonomous measurement gaps, in E-UTRAN—the type of RAN used in LTE networks—the serving cell can request the UE to acquire the Cell Global Identifier, CGI, of a given target cell. CGIs uniquely identify cells within a network. To acquire the CGI of a target cell, the UE has to read at least part of the System Information, SI, of the target cell, including the Master Information Block, MIB, and the relevant System Information Block, SIB, as described later.
Acquiring the CGI of a target cell involves reading the SI of the target cell during measurement gaps, which are autonomously created by the UE. An “autonomous measurement gap” is a gap in reception at the UE, a gap in transmission at the UE, or a gap in both reception and transmission at the UE. A UE creates such gaps at a point in time determined by the UE, for instance to allow time for reconfiguration of its radio circuitry to acquire system information from a neighbor cell or to perform other kinds of signal measurements. Reconfiguration includes, for example, tuning to another cell or frequency.
According to Section 5.5.3.1 of 3GPP Technical Specification (TS) 36.331, version 12.7.0, a UE reads the MIB and the SystemInformationBlockType1 or SIB1 of a target cell, to obtain the CGI of the target cell. The term “E-CGI” or “ECGI” is used rather than “CGI”, when the target cell is an E-UTRAN intra- or inter-frequency cell.
In LTE, FDD mode acquisition of the MIB and SIB1 by a UE for a target cell assumes that the UE needs to perform Automatic Gain Control, AGC, on the target cell carrier before reading the MIB and SIB1. E-UTRA FDD MIB and SIB1 acquisition. See, e.g., 3GPP TS 36.133 version 12.7.0. A further assumption is that four subframes may have to be blanked—interrupted or ignored—for acquiring each of the target cell MIB blocks and SIB1 Redundancy Versions, RV. Still further, it is assumed that three blocks of the MIB and four RVs are needed from the SIB1, for the same 40 and 80 ms period, respectively. For acquiring each of the MIB and the SIB1, five gaps with a duration of 4 ms each are allowed. One of the five gaps may be permitted to be 5 ms, in view of AGC/AFC operations. FIG. 5 illustrates these details, where B1, B2, B3 and B4 denote blocks of the Physical Broadcast Channel or PBCH.
Further in LTE, the MIB includes a limited number of the most essential and the most frequently transmitted parameters that are needed to acquire other information from the cell, and is transmitted on the Broadcast Control Channel, BCH, of the target cell. In particular, the following information is currently included in MIB: DL bandwidth, Physical Hybrid Automatic Repeat reQuest, HARQ, Indicator Channel, PHICH, configuration, and System Frame Number or SFN. The MIB is transmitted periodically with a periodicity of 40 ms and repetitions are made within 40 ms. The first transmission of the MIB is scheduled in subframe #0 of radio frames for which the SFN mod 4=0, and repetitions are scheduled in subframe #0 of all other radio frames.
In LTE the SIB1 contains the following information: Public Land Mobile Network, PLMN, identity; Cell identity; Close Subscriber Group, CSG, identity and indication; Frequency band indicator; SI-window length; and scheduling information for other SIBs transmitted in the target cell. The LTE SIB1 may also indicate whether a change has occurred in the SI messages. A UE is notified about coming changes in the SI by a paging message, from which the UE recognizes that the SI for the cell will change at the next modification period boundary. The modification period boundaries are defined by SFN values for which (SFN mod m)=0, where m is the number of radio frames comprising the modification period. The modification period is configured by system information.
The LTE SIB1 is transmitted on the DL Shared Channel or D-SCH, as are the other SIBs. The SIB1 is transmitted with a periodicity of 80 ms and repetitions made within 80 ms. The first transmission of SIB1 is scheduled in subframe #5 of radio frames for which the SFN mod 8=0, and repetitions are scheduled in subframe #5 of all other radio frames for which SFN mod 2=0.
In case of inter-RAT UTRAN, the UE reads the MIB and SIB3 of the target UTRAN cell to acquire its CGI. More generally, the UE may perform measurements in autonomous gaps to determine the CGI of a given target cell on the same frequency as the serving cell(s) of the UE and having the same RAT as the serving cell(s). However, the target cell may be on a different frequency and/or operate according to a different RAT, and the gap-based measurements therefore may be inter-frequency and/or inter-RAT.
A number of well-known scenarios exist where the serving cell of a UE requests that the UEs report the CGI of a target cell. These scenarios include: verification of a CSG cell; establishment of Automatic Neighbor Relations, ANR, in a Self-Organizing Network, SON, context; Minimization of Drive Test, MDT, operations; and verification of CSG cell for CSG inbound mobility.
To support mobility within the network, a UE is required to identify a number of neighbor cells and report their Physical Cell Identities, PCIs, to the serving network node. The serving network node is, for example, a serving eNB in the E-UTRAN. The UE may also be requested to report measurement results for one or more of the neighbor cells, such as RSRP and/or RSRQ in E-UTRAN or CPICH RSCP and/or CPICH Ec/No in UTRAN, GERAN carrier RSSI, or pilot strength for CDMA2000/HRPD, where “HRPD” denotes High Rate Packet Data. The serving network node uses the measurements reported by the UE to make handover decisions with respect to the UE, for example.
Dense deployment scenarios involve smaller cell sizes and the PCIs are more frequently reused. Small cell examples include “femto” or “pico” cells, and broadly encompass the use of CGS cells, such as with home base stations, etc. To avoid commanding the UE to move from its current serving cell to a restricted cell, e.g., a cell subject to CSG membership, the serving network node may request that the UE decode and report the target cell CGI. Because CGIs are unique within a network, having the target cell CGI enables the serving network node to determine whether the target cell corresponds to a macro base station or other non-CSG access point, or corresponds to an access point having CSG restrictions.
Specifications related to E-UTRAN define CGI reporting procedures. A key aspect of CGI decoding is that CGI determination is performed by a UE during autonomous measurement gaps created by the UE. This arrangement arises from the fact that the typical UE is incapable of simultaneously receiving data from the serving cell while acquiring the SI of a target cell. Furthermore, CGI acquisition in inter-frequency or inter-RAT contexts requires the UE to switch carrier frequencies. Hence, autonomous gaps are inevitably required for a UE to obtain the SI of a target cell. The autonomous gaps are created in both the DL and the UL.
Another usage of autonomous measurement gaps at a UE involves ANRs in the SON context. To ensure correct establishment of neighbor cell relations, a serving cell requests that a UE report the CGI of a new target cell, whose PCI is identified and reported to the serving cell. The CGI acquisition requires the UE to acquire the SI of the target cell, and is thus carried out by the UE during the autonomous gaps. CGI acquisition for ANR purposes therefore also leads to interruption of data reception at the UE with respect to the serving cell.
Further, Release 10 of the 3GPP specifications for LTE and High Speed Packet Access, HSPA, introduced MDT features aimed at reducing the effort needed to gather network coverage and performance information, such as is used for network planning and optimization. The MDT feature requires that the UEs log or obtain various types of measurements, events and coverage related information. The logged or collected measurements or relevant information are then sent to the network. This approach contrasts with traditional approaches in which the network operator collects similar information using actual drive tests and associated manual logging.
The 3GPP TS 37.320 V.10 describes the MDT feature in more detail, but it may be helpful to note some of the measurements that may be made or collected by a UE operating in a network in a connected mode or at least in certain low-activity states. For example, the UE may measure, log and subsequently report: mobility measurements, e.g., RSRP, RSRQ etc.; random access failures; paging channel failures; broadcast channel failures; and radio link failures.
A UE can also be configured to report the CGI of target cells along with other measurements, such as RSRP, RSRQ, etc. Existing measurement procedures are used to obtain the CGI of a target cell for MDT purposes, in cases where the UE is in the connected mode. With respect to idle mode operation, the UE can be configured to log the cell measurements along with the corresponding CGIs, and subsequently report the logged measurements to the network at a suitable occasion, such as when the UE goes to connected mode. CGI acquired for MDT purposes is based on the UE making the required measurements during autonomous measurement gaps.
The use of carrier aggregation, CA, involves certain considerations regarding the use of autonomous measurement gaps by a UE, for acquiring target-cell SI. In a CA configuration example, a UE is configured to have a Primary Cell or PCell and a Secondary Cell or SCell. In this context, the UE applies SI acquisition and SI-change monitoring procedures for the PCell only. For SCells, E-UTRAN provides, via dedicated signaling, all SI relevant for operation in RRC_CONNECTED mode when adding the SCell. Hence, the UE creates autonomous gaps on the PCell DL and UL, for reading a neighbor-cell CGI. E-UTRAN therefore specifies a number of SI/CGI acquisition requirements for the following scenarios: intra-frequency CGI reporting, where CGI is denoted as E-CGI in the E-UTRAN context; inter-frequency E-CGI reporting; and inter-RAT UTRAN CGI reporting.
The UE is required to report intra-frequency E-CGI for a target intra-frequency cell within about 150 ms, provided that the Signal-to-Interference-plus-Noise Ratio, SINR, for the target cell at the UE is at least −6 dB or higher. During acquisition of the E-CGI for the target cell on the serving carrier frequency, the UE is allowed to create autonomous gaps in the DL and UL. Under continuous allocation, the UE is required to transmit a certain number of Acknowledgments/Non-Acknowledgments, ACKs/NACKs, on the UL. This requirement ensures that the UE does not create excessive gaps.
The UE is also required to report the inter-frequency E-CGI within about 150 ms from a target inter-frequency cell, provided that the SINR of the target cell at the UE is at least −4 dB or higher. During acquisition of the E-CGI for the target cell on the target carrier frequency, the UE is allowed to create autonomous gaps in the serving cell DL and UL. These gaps represent interruptions in DL reception and UL transmission at the UE with respect to the serving cell. However, when the UE has a continuous DL allocation in the serving cell, the UE is required to transmit certain number of ACK/NACK on the serving-cell UL. This requirement ensures that the UE does not create excessive gaps.
The minimum number of ACKs/NACKs that the UE is required to send under continuous DL allocation is specified as a requirement that UE has to meet. The minimum number of ACK/NACK transmissions by the UE for E-UTRA FDD is 60 ACKs/NACKs. For E-UTRA TDD, the minimum number of ACKs/NACKs depends upon the UL-DL TDD configuration. For example, the UE is required to send eighteen ACKs/NACKs for UL-DL TDD configuration #0, and thirty ACKs/NACKs for UL-DL TDD configurations #1.
In UTRAN, acquisition of the CGI for a target cell takes much longer, e.g., longer than one second, with the actual amount of time depending upon the periodicity of the SIB3 transmissions in the target cell. The SystemInformationBlockType3 or SIB3 contains the CGI. These circumstances mean that the UE may interrupt its serving cell data transmissions and receptions for 600 ms or longer, when acquiring the CGI of a UTRAN target cell.
LTE imposes no requirements on UEs regarding the acquisition of CGI for an E-UTRAN cell in parallel with performing other intra- or inter-frequency or inter-RAT measurements. Examples of such other measurements are intra-frequency cell search, RSRP, RSRQ, radio link monitoring or inter-frequency cell search, RSRP, RSRQ or inter-RAT UTRAN cell search, CPICH measurements, etc. Yet other examples are the positioning measurements such as UE Receive-Transmit, Rx-Tx, time difference measurements, TA measurements, eNB Rx-Tx time differences, etc. These positioning-related measurements require measurements on signals transmitted in the UL.
In general, the use of autonomous measurement gaps by the UE may adversely affect the requirements of such other measurements. In particular, the autonomous gaps in the UL affect positioning measurements that involve or rely on the measurement of UL signals transmitted by the UE.
The use of autonomous measurement gaps also affects “Dual Connectivity” or DC operation of a UE. With DC, a UE can be served by two eNBs simultaneously, with one eNB referred to as the Master eNB, MeNB, and the other eNB referred to as the Secondary eNB, SeNB. Further, the UE is configured with a Primary Component Carrier or PCC, for both the MeNB and the SeNB. The PCC of the MeNB is associated with a Primary Cell or PCell, and the PCC of the SeNB is associated with a Primary Secondary Cell or PSCell. The PCell and PSCell typically operate independently with respect to the UE. FIG. 6 illustrates a number of UEs operating in DC configurations—i.e., having both a MeNB and a SeNB.
The DC configuration also includes one or more Secondary Component Carriers or SCCs from each of the MeNB and the SeNB. The SCCs are associated with corresponding secondary serving cells, referred to as SCells. Other DC-related terminology includes the term Master Cell Group or MCG, and Secondary Cell Group or SCG. The MCG denotes the PCell and the SCells, if any, of the MeNB, while the SCG denotes the PSCell and the SCells, if any, of the SeNB.
A UE equipped for DC operation typically has separate transmitter and receiver resources to support each of the two connections—i.e., transceiver circuitry for its radio links with the MeNB and transceiver circuitry for its radio links with the SeNB. This separation allows the MeNB and the SeNB to independently configure the UE with respect to certain radio procedures involving the PCell and the PSCell, such as radio link monitoring, Discontinuous Reception, DRX, cycles, etc.
Two operational modes are considered in the context of DC, with the first being implemented in Release 12 of the 3GPP specifications for E-UTRA, and with the latter to be specified in a later release. The first mode is referred to as “synchronized operation”, where DL timing at the UE for the MeNB and the SeNB is synchronized down to about half an OFDM symbol, which in LTE is about ±33 μs. Thus, in synchronized DC operation, the time difference, Δτ, between signals received at the UE from the serving cells of the MeNB should be aligned in time within ±33 μs of the signals received at the UE from the serving cells of the SeNB. For the unsynchronized mode of operation, DL timing at the UE for the MeNB and the SeNB is synchronized down to half a subframe, which is ±500 μs in the LTE context.
More generally, one may define synchronized operation in the DC context as the case where the received signal time difference Δτ is within a first range capped by a time value, denoted as threshold Γ1. Similarly, one may define unsynchronized operation in the DC context as the case where the received signal time difference Δτ is within a second range capped by a time value, denoted as threshold Γ2. Thus, synchronized operation applies where Δτ<Γ1, and unsynchronized operation applies where Γ1≤Δτ<Γ2. Operation also may be deemed to be unsynchronized if Δτ is allowed to have any arbitrary value.
FIG. 7 illustrates example synchronized and unsynchronized cases. In the diagram, “MRTD” denotes “maximum received signal time difference” and should be understood as corresponding to the above-mentioned Δτ.
DC operation contemplates the following duplex mode configurations: MeNB FDD, SeNB FDD; MeNB FDD, SeNB TDD; MeNB TDD, SeNB FDD; and MeNB TDD, SeNB TDD. Moreover, UL/DL configurations for TDD cells may be different for different carriers. With momentary reference to FIG. 3, it may be noted that the GP of special subframes seen in the TDD mode of operation may be different as between the communication links used in one connection and the communication links used in the other connection, to account for different timing advance values being used at the MeNB and at the SeNB with respect to the UE. The difference between GP length as between cells is mainly due to differences in timing advance. To achieve a particular timing advance, GP may be reduced accordingly. The net effect in the blanking context is that the whole gap shrinks because the timing advance value also dictates the transmission timing for uplink subframes immediately following a measurement gap.
However, it is currently not specified how a UE should carry out acquisition of SI for a target cell when the UE is configured for DC operation. If conventional approaches hold, the UE in question would apply its configured measurement gaps uniformly to the MeNB carriers and the SeNB carriers. In other words, the MeNB carriers would be interrupted or “blanked” for the same duration(s) as the SeNB carriers.
It is recognized herein that symmetrical blanking by a UE of the MeNB and SeNB carriers does not account for special circumstances, requirements and opportunities for improved operation in the context of DC operation. For example, it is recognized herein that in dual connectivity operation, the SFN between cells of the MCG and the SCG is not aligned in the most general case. Thus, at any given time, the SFN of the cells in the MCG may not be the same as the SFN of the cells in the SCG.
It is further recognized herein that the application of measurement gaps by the UE is complicated in the DC case, because the subframe boundaries can be time misaligned as between the MCG cells and the SCG cells. For example the transmit timings of a subframe in the MCG cells may be shifted in time by as much as 0.5 ms in relation to the corresponding timings in the SCG cells. Consequently, the imposition of a measurement gap by the UE on any given CC involved in its DC configuration may impact other involved CCs in an unpredictable manner. In other words, it is recognized herein that symmetrical or uniform blanking of the CCs involved in DC or other “multi-connectivity” operating configurations is undesirable, at least under certain circumstances.